It is common to dry a material such as an aggregate in a rotating drum. Typically, wet material is introduced into the drum. A burner often in conjunction with a blower form a flame within the drum. The flame heats and dries the material as it moves along the interior of the drum. Dry material exits the drum at a location remote from the inlet location. However, in the past, there have been several problems associated with drying materials in such drums.
One common problem with drying materials in drums has been that material falls into the flame during the drying process. The material is initially introduced into the drum at or near the bottom of the drum. However, because the drum is rotating, the material within the drum slowly rotates up the side wall(s) of the drum. Eventually, gravity and other forces cause the material to fall in a downward direction from the side wall(s) of the drum. When the material falls in the downward direction, it occasionally passes through the flame. When material falls into the flame, it causes the undesirable effect of quenching or partially quenching the flame. Further, the hydrocarbons and other constituents within the partially combusted fuel may form smoke or other unwanted residue. In addition to environmental emissions concerns, this affects the efficiency of the drying system.
Another common problem with drying materials in drums has been that the material is not evenly dried. The material has a certain depth as it is introduced into the drum. As the drum rotates, the material is not significantly stirred. Thus, the material at or near the surface faces the flame and dries more quickly. The material at or near the surface shields the under layers of material from the heat of the flame. As a result, the under layers of material often do not dry completely.
A further problem in many existing drying drums is that the material does not advance evenly through the length of the drum. The material moves longitudinally relative to the drum as the drum rotates. When the material advances unevenly, it further worsens the problem that the material is not evenly dried. Also, the material is often very abrasive. When the material does not evenly advance, it often causes substantial wear on the drum's inner side wall surface. The drum itself is very expensive to replace.
One solution to the problem of material falling into the flame has been the use of specially designed interrupting veiling flights such as “T” flights. Interrupting veiling flights such as “T” flights are typically affixed to the inner surface of the drum. For example, interrupting veiling flights have been designed to grab material from near the bottom of the drum. The material grabbed by the interrupting veiling flights is dropped as the drum rotates. However, the material grabbed by the flights is veiled and dropped before and after it has been lifted over the hot portion of the flame. When the flight is over the hot portion of the flame, the flight works to prevent material from falling. The interrupting veiling flights also absorb the heat from the flame and shield the material being held in the flight from the flame. There are several drawbacks to using interrupting veiling flights in this manner. For example, the interrupting veiling flights often lock or hold patches of material in the spaces between the flights. The locked material is not evenly dried. The interrupting veiling flights frequently do not assist in controlling the flow or advancing the material through the length of the drum.
The present invention is provided to solve the problems discussed above and other problems, and to provide advantages and aspects not provided by prior drums of this type. A full discussion of the features and advantages of the present invention is deferred to the following detailed description, which proceeds with reference to the accompanying drawings.